In an autonomous pedestrian navigation system that uses a pedestrian dead reckoning (PDR) technique, in which a position of a person or other object is detected and then a trajectory of the person or object is monitored or tracked based on detected changes in position. In the PDR technique, a moving direction and a moving distance can be detected by an acceleration sensor, a gyro sensor (also referred to as an angular velocity sensor), or a geomagnetic sensor (also referred to as an electronic compass).
In the PDR technique, a position of the person or object is measured relative to an arbitrary base point. That is, the position is represented according to a relative coordinate system. Therefore, for example, when the trajectory of person or object is to be displayed on a map using an absolute coordinate system, it is necessary to convert positions in the relative coordinate system (also referred to as relative coordinates) into coordinates in an absolute coordinate system (also referred to as absolute coordinates) for representation on the map.
In general, a person or an object carries a mobile station such as a mobile phone or mobile computer that can be connected to a mobile network. In communicating with the mobile network, the mobile station receives radio waves transmitted from a fixed-location transmission station. When the radio waves are received at the mobile station, the absolute coordinates of the transmission station can be used in determining the current position of the mobile station carried by the person or object. For example, the positioning device obtains a position in the absolute coordinate system, that is, a so-called absolute position, from a position measurement result obtained by a PDR technique that uses the coordinates of the fixed-location transmission station as a base point.
However, there may be inaccuracies in determining coordinates representing the current position of the mobile station in this manner. When a distance that radio waves travel from the transmission station to the mobile station is long, there is a possibility that the coordinates of the mobile station specified in the radio waves from the transmission station may not correspond to an actual position of the mobile station at the time of receiving the radio waves. That is, an absolute position of the mobile station in the absolute coordinate system set at the transmission station specified in the received radio wave is deviated from an actual position of the mobile station, and such deviations become large as a distance between the mobile station and the transmission station becomes large. However, if the permissible distance between the mobile station and the transmission station is shortened or restricted, then there may be instances in which the mobile station cannot utilize radio waves from a fixed-location transmission station and thus the current coordinates of the mobile station cannot be specified at all times.
In the related art, there is a technique in which a predetermined operation is performed by the mobile station when the mobile station comes closest to the transmission station, and the absolute coordinates are set according to signals received from the fixed transmission station after the predetermined operation is completed. Also, there is a technique in which a threshold value of the received signal strength from the transmission station is set in advance within the mobile station and the absolute coordinates from the fixed transmission station are set according to when the received signal strength exceeds the threshold value. However, the first technique is cumbersome because, in general, a non-autonomous operation by a person is required and thus use is restricted because human intervention is required. In the second technique, the received signal strength from the fixed transmission station may exceed the threshold value at an unexpected place distant from the fixed transmission station and in such a case, accuracy of positioning will be decreased.